My guess is that the car was never even on the carrier it the first place. I bet the car was just parked behind the rig on the side of the street and for some reason, the transmission was not put into park. You can see that there is a slight downhill incline towards the carrier and the right ramp lines up with where the left wheels would be on a car parked on the side of the street. The car simply rolled onto the right ramp with the left wheels and flipped over just like on the setups used in movies and stunt shows.